If you're playing Don don't despair! Don is as important as everyone else on the line. He may not be as featured as some of the other performers, but you have to look at the show as "The Don Kerr Show" and create an interior monologue for the entire musical. Let the audience know who you are through your reactions if you don't have a line. Unfortunately, if
everyone got a full solo, the show would be five hours long, and that just wouldn't work.
Don Kerr was based on a mixture of Andy Bew, Ron Kuhlman (original Don), and Michael Bennett. Andy was the "type" of person who inspired the character, Ron's personality was added, and the stripper story is inspired by Michael Bennett's life.
Don is kinda a ladies man, a player, but is married. He's jazzy and cool, but a little egotistical as well. He's a hedonist, really into cars, money, and women. His lines in the Montage help illustrate this. Supposedly, he had an affair with Sheila, although I've only heard this, not actually read this anywhere.
One scene that was cut from the show was a scene between Sheila and Don where he told about how his marriage was falling apart. Keep this in mind when you talk about your family in the Alternatives scene. And although, the movie is not "official" I think there's a lot of interesting characterization for Don regarding his family and day job.
Another thing that was cut for Don was that he originally had a long monologue about the death of his mother. Andy Bew was in the original touring company of "Hello Dolly" and his mom was hired as a chaperone (He was only 16 at the time) Anyway, they were in Chicago, and while they were boating, the boat capsized and his mom drowned in Lake Michigan. So, he was basically on his own, and the theater really became his family. He tried to go back to his
hometown after the tour ended, but he had money, bought a car, had seen the country, and his friends were just...small town. So, he moved to New York and never looked back.
Ron Kuhlman was hired to add some singing power to the line and isn't necessarily one of the best dancers. Don is supposed to be "jockish" and have a lot of charm. Play around with a "signature" greeting (like a tcht or wink or something with your hands) that you can use to differentiate yourself. This helps Don stand out as well.
Other facts: Don is the first person stage right. He typically wore jeans, sneakers, and a blue or pink and black striped shirt. He has been in the business for 10 years, and would know a lot of people on the line. Supposedly, he knows Maggie, Mike, Sheila, and others. You might consider that maybe he's making the moves on Maggie, and that's why he doesn't know what to say when he has to introduce himself.
Don's biggest competition is probably Mark, since they have a similar All-American look (at least originally, it would depend on your cast). That could be interesting to play up.
Ron Kuhlman went on to play the husband of Jan Brady on tv, if you've ever seen those specials. Andy Bew had worked with Michael Bennet before, so if you use that for Don, that might be another reason he could be cocky.